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There is even the same ankylosis of the neck vertebrae. We
find, moreover, the same association of long-legged and shorter-
legged forms that characterises the Heteromyidae.

The typical genus Dipus is a smallish quadruped with long
naked ears and a long tail. The ten
species are all Palaearctic in range. The
fore-limhs are short and five fingered, and
the short pollex has no claw; the hind-
limbs are excessively long and only three-
toed. The bony structure of these limbs
is remarkable. The three metatarsals are
elongated almost like those of a bird, and
are ankylosed together. The digits have
long phalanges which alone reach the
ground as the animal hops. It is a curious
fact, and one not so easily identifiable with
the way of life, that the neck vertebrae of
this genus are ankylosed together with the
exception of the atlas, which, is free; the
arrangement is precisely like that of the
Sperm Whale. The last vertebra is, how-
ever, sometimes free.

only leap but they burrow, and their strong
incisors are said to be used in burrowing
through stony ground. They are eaten by
the Arabs, and are, or have been, called
Daman Israel, i.e. I^amb of Israel In
-£?. Tiirtipes the body and tail measure respectively 4-|- and 7
inches. The hind-feet have a tuft of long hairs below, Mr.
W. L. Selater's newly-founded genus Euchoreutes1 is somewhat
more primitive in its characters than is Diyus. The general
form is the same, -with long ears and a long talL But there
are five toes to the hind-limb, the two lateral ones though
nailed being much shorter than the middle three. It has a
<c long pig-like snout/* and the tail is cylindrical as in most other
Jerboas, with a tuft of longer hairs at the end. The incisor
teeth, grooved in Dipus^ are here smooth, as in ^Alactaga^ The
species was probably obtained * in the sandy plains round the
city of TTarkand/*

1 Pro*, Zool, Soc* 1890, p. 61O.

Th*>    TVrbAfl*    -not    FIG. 239.—Bones of right p«s
±ne   tierDOas    nOU        of Jerboa, IHgnuaeffypttus.
xf. <z, Astragalus; c,
calcaneurn; c2, middle
cuneiform; c3, outer
cuneiform; c&, cttboid;
n, navicialar; I-IV, first
to foxirtli toes, (Froia
Flower's Osteology.)